1. Field of the Invention
The present invention relates to a lithium secondary battery, and more particularly, to a secondary battery which has an improved structure by which the safety of the battery is improved.
2. Description of the Related Art
In general, secondary batteries are capable of charging, unlike primary batteries which are incapable of charging and are widely applied to portable electronic apparatus such as cellular phones, notebook-type computers, camcorders and the like. In particular, since the lithium secondary batteries have a service life three times longer than Ni-Cd batteries or Ni-MH batteries and excellent energy density per unit weight, they are in widespread use.
Lithium secondary batteries are classified into liquid electrolyte batteries and polymer electrolyte batteries according to the type of electrolyte used. Generally, batteries using a liquid electrolyte are called lithium ion batteries and batteries using polymer electrolyte are called lithium polymer batteries.
Here, lithium secondary batteries may have various shapes. Typically, cylindrical or prismatic batteries are fabricated to be used mainly as lithium-ion batteries. Lithium polymer secondary batteries have flexibility so that they are relatively free in view of shape design. Accordingly, lithium polymer secondary batteries having excellent safety and freedom in shape design and being light-weight are advantageous in attaining miniaturized and light-weight portable electronic apparatus, and research into the lithium polymer secondary battery is being carried out in various manners.
FIG. 1 is a perspective view illustrating an electrode assembly of a conventional lithium secondary battery.
Referring to FIG. 1, the electrode assembly includes has a positive electrode plate 11 and a negative electrode plate 12 stacked sequentially, with a separator 13 being interposed therebetween. A positive electrode tab 14 and a negative electrode tab 15 are drawn out at each one side of the positive and negative electrode plates 11 and 12, respectively. The positive electrode plate 11 includes a positive electrode current collector 16 made of aluminum foil and a positive electrode sheet 17 fixed to at least one surface of the positive electrode current collector 16 and having a positive electrode active material as a main component, a binder, a conductive material and a plasticizer. In FIG. 1, the positive electrode sheet 17 is fixed to both surfaces of the positive electrode current collector 16.
Also, like the positive electrode plate 11, the negative electrode plate 12 includes a negative electrode current collector 18 made of copper foil and a negative electrode sheet 19 fixed to at least one surface of the negative electrode current collector 18 and having a negative electrode active material as a main component, a binder, a conductive material and a plasticizer. In FIG. 1, the negative electrode sheet 19 is fixed to both surfaces of the negative electrode current collector 18.
In the electrode assembly having the aforementioned configuration, the positive electrode sheet 17 and the negative electrode sheet 19 are fused to the positive electrode current collector 16 and the negative electrode current collector 18 in a laminating process. After fusion, the plasticizer is extracted from the positive and negative electrode plates 11 and 12 and then an electrolyte is impregnated into the space produced by extracting the plasticizer, thereby fabricating the electrode assembly.
The lithium polymer secondary battery having the above-described electrode assembly is fabricated in a packaged state, and a safety device is used in each battery cell and the package. The safety device protects the battery by cutting off the current in an event of a rise in the battery temperature or the internal pressure, or by discharging the pressure outside of the battery.
The rise in the battery temperature or pressure may be caused by overcharging or external shorts. In order to ensure safety, various types of lithium secondary batteries are being currently developed. However, there have never been safety devices adapted to positive and negative electrode plates in conjunction with battery safety.
To solve the above problems, it is an object of the present invention to provide a secondary battery having an improved structure in which each current controller for ensuring the battery safety is installed on positive and negative electrode plates.
A further lithium secondary battery according to the invention includes an electrode assembly, the electrode assembly comprising a positive electrode plate having an inner surface and an outer surface, a positive electrode current collector having first and second surfaces, a positive electrode sheet fixed to at least one of the first and second surfaces of the positive electrode current collector and having a positive electrode active material as a main component, and first and second layers of a positive temperature coefficient material that cuts off current flow when the positive temperature coefficient material reaches at least a predetermined temperature, coating opposite sides of the positive electrode current collector and defining the inner and outer surfaces of the positive electrode plate, respectively; a negative electrode plate having an inner surface and an outer surface, a negative electrode current collector having first and second surfaces, a negative electrode sheet fixed to at least one of the first and second surfaces of the negative electrode current collector and having a negative electrode active material as a main component, and third and fourth layers of the positive temperature coefficient material coating opposite sides of the negative electrode current collector and the negative electrode sheet and defining the inner and outer surfaces of the negative electrode plate, respectively; and a separator interposed between and contacting the inner surface of the positive electrode and the negative electrode plate.
A further lithium secondary battery according to the invention includes an electrode assembly comprising a positive electrode plate having an inner surface and an outer surface, a positive electrode current collector having first and second surfaces, a positive electrode sheet fixed to at least one of the first and second surfaces of the positive electrode current collector and having a positive electrode active material as a main component; first and second layers of a positive temperature coefficient material that cuts off current flow when the positive temperature coefficient material reaches at least a predetermined temperature, disposed on opposite sides of the positive electrode current collector and defining the inner and outer surfaces of the positive electrode plate, respectively; and a negative electrode plate having an inner surface and an outer surface, a negative electrode current collector having first and second surfaces, a negative electrode sheet fixed to at least one of the first and second surfaces of the negative electrode current collector and having a negative electrode active material as a main component, and third and fourth layers of the positive temperature coefficient material disposed on opposite sides of the negative electrode current collector and defining the inner and outer surfaces of the negative electrode plate, respectively; and a separator interposed between and contacting the inner surfaces of the positive and negative electrode plates.